Although it is known that CNS glycogen is located exclusively in astrocytes its function is not well understood. Although an obvious role for glycogen is as an energy source only two studies have addressed this. In one study neurons grown in astrocyte-rich cultures are less severely injured by aglycaemia than neurons in astrocyte-poor cultures (Swanson & Choi, 1993). In the second study we showed that maintenance of axon function in CNS white matter during aglycaemia was dependent upon the presence of glycogen, and that glycogen in astrocytes was metabolized to lactate, which was shuttled to axons where it was used as an energy source (Wender et al. 2000). We report further studies on the role of glycogen during hypoglycaemia.

Adult Swiss Webster mice were deeply anaesthetized with CO₂ in an enclosed chamber, and exsanguinated by decapitation. This procedure is within the guidelines laid down by the University of Washington animal welfare committee. Optic nerves were placed in an interface perfusion chamber in ACSF maintained at 37 °C, bubbled with 95 % O₂ and 5 % CO₂, and allowed to recover for 1 h in 10 mM glucose. Axon function was assessed by monitoring the compound action potential (CAP), which was evoked every 30 s by a 50 µs supramaximal stimulus. Glycogen content was measured as previously described (Wender et al. 2000). Data are presented as means ± S.E.M. and one-way ANOVA tests were carried out to determine significance.

CAP function was fully maintained in ACSF containing 10 mM glucose (n = 3) or 20 mM lactate (n = 3). The glycogen content of the nerve after the 1 h recovery period (control glycogen level) was 6.54 ± 0.28 pmol glycogen (µg protein)^-1 (n = 4). After a further 2 h in 10 mM glucose the glycogen content was not significantly different than control (6.57 ± 0.23 pmol glycogen (µg protein)^-1, n = 3, P ▓ge│ 0.05). Decreasing glucose to 2 mM did not affect the ability of the CAP to be maintained at maximum amplitude for 2 h (n = 3), but glycogen content fell significantly to 2.34 ± 0.20 pmol glycogen (µg protein)^-1 after 2 h (n = 3, P ▓le│ 0.001 compared with control). Given that 2 mM glucose is considered hypoglycaemic we wondered whether glycogen was contributing to maintenance of axon function. We depleted glycogen by introducing glucose-free ACSF for 15 min before introduction of 2 mM glucose ACSF. This resulted in glycogen falling significantly to 2.30 ± 0.41 pmol glycogen (µg protein)^-1 (n = 3, P ▓le│ 0.001 compared with control). ACSF containing 2 mM glucose was unable to fully support axon function after glycogen depletion and CAP area fell to 26.3 ± 1.6 % of baseline (n = 3). In the presence of the lactate transport inhibitor cinnamic acid, 2 mM glucose was unable to fully support axon conduction. We conclude that in the adult mouse optic nerve astrocytic glycogen acts as a readily available energy source to support axon function during periods of hypoglycaemia.This research was supported by the National Institute of Health Grant 15589 (B.R.R.) and the Eastern Paralyzed Veterans Association (A.M.B. and B.R.R.).

Swanson, R.A. & Choi, D.W. (1993). J. Cereb. Blood Flow Metab. 13, 162-169.

Wender, R., Brown, A.M., Fern, R., Swanson, R.A., Farrell, K. & Ransom, B.R. (2000). J. Neurosci. 20, 6804-6810.